1. Field of the Invention
The present disclosure relates to a nitride semiconductor device applicable to a power device included in a power supply circuit of a television set or other consumer products.
2. Description of the Related Art
A nitride semiconductor, including a GaN semiconductor as a typical example, has a wide gap. For example, GaN and AlN semiconductors have wide band gaps of 3.4 eV and 6.2 eV at room temperature, respectively. Accordingly, a nitride semiconductor is characterized by a higher breakdown electric field, and a higher saturated electron drift velocity than that velocity of a compound semiconductor such as a GaAs semiconductor or of a Si semiconductor. In AlGaN/GaN heterostructure, charges are generated at a heterointerface as a result of spontaneous polarization and piezoelectric polarization on (0001) plane. In this case, a sheet carrier concentration of 1×1013 cm−2 or higher is obtained even in an undoped state. Accordingly, such diode and hetero-junction field effect transistor (HFET) which have higher current density are producible by use of two-dimensional electron gas (2DEG) at a heterointerface. In recent years, therefore, research and development of a power device utilizing a nitride semiconductor have been actively conducted in view of advantages of the nitride semiconductor capable of realizing higher output and higher breakdown voltage.
The “AlGaN” noted herein represents ternary mixed crystal AlxGa1-xN (x: arbitrary value, 0≦x≦1). Hereinafter, multi-element compounds are expressed simply by the arrangement of chemical symbols such as AlInN, GaInN, etc. For example, nitride semiconductor AlxGa1-x-yInyN, where 0≦x≦1, 0≦y≦1, and x+y≦1, is expressed simply by AlGaInN.
A Schottky diode is an example of a main device included in GaN power devices. Since this diode uses a channel produced by two-dimensional electron gas generated at an interface between an undoped AlGaN layer and an undoped GaN layer laminated on each other, large-current and low-resistance operation is achievable.
In general, a Schottky diode provides advantages such as an excellent switching behavior, and a low forward threshold voltage. However, a Schottky diode has a disadvantage of large backward leakage current. For overcoming this drawback, a method currently proposed inserts a p-GaN layer into an anode electrode formed on an undoped AlGaN layer, for example, so as to reduce interface leakage current. This method produces a diode capable of reducing leakage current at a low operating voltage.